Enamel is one of the major tissues within teeth. It is formed during early development, before the tooth breaks through the gum, and it covers the visible portion of the tooth. During development, epithelial cells within the enamel organ differentiate into ameloblasts, which produce the hardened, high mineral-content enamel. More specifically, the enamel organ appears naturally in vivo as an aggregate of cells in histologic sections of the developing tooth, and it includes the inner enamel epithelium, where ameloblasts reside, the outer enamel epithelium, the stratum intermedium, and the stellate reticulum. The dental organ is comprised of the enamel organ and mesenchyme.
Maintaining and repairing enamel are primary concerns for dentists and others working in the field of dentistry. This is challenging because enamel demineralizes in the mouth and does not regenerate. Although mechanisms for remineralizing teeth are known, the state of equilibrium can easily be tipped toward demineralization. For example, bacteria within the dental plaque (e.g., Streptococcus mutans) can produce organic acids that, particularly in the presence of sugars, reduce the pH in the mouth, promoting demineralization and dental caries. Engineering enamel is obviously desirable, but this has proven difficult due to not only a lack of ameloblasts in erupted teeth, but also a developmental requirement; thus far, the epithelial cells that can differentiate into ameloblasts must interact with mesenchymal cells to develop properly. Previous efforts to produce enamel have included both cell-based and cell-free strategies. Commonly, cell-based strategies rely on the development of a stable cell line that is combined with mesenchymal cells in Matrigel™ (a gelatinous and protein-containing cell culture substrate) and transplanted into an animal where enamel is generated in vivo (see, e.g., DenBesten et al., Connect Tissue Res. 38(1-4):3-8, 1998; DenBesten et al., Eur. J. Oral Sci., 107(4):276-81, 1999; Nakata et al., Biochem. Biophys. Res. Commun., 308(4):834-839, 2003; Honda et al., Cells Tissues Organs 189(1-4):261-267, 2009; Shinmura et al., J. Cell. Physiol. 217(3):728-738, 2008; Nakagawa et al., J. Dental Res. 88(3):219-223, 2009; Takahashi et al., In Vitro Cell Dev. Biol. Anim., 46(5):457-468, 2010; Liu et al., J. Tissue Eng. Regen. Med. 7:934-943, 2012; Hu et al., J. Dent. Res., 85(5):416-421, 2006; and Chen, Arch. Oral Biol. 37: 771-778, 1992). As enamel contains about 97% hydroxyapatite and has no cells, synthetic processes are also possible (see, e.g., Yamagishi et al., Nature 433:819, 2005; and Brunton et al., Br. Dental J. 215:741, 2013). Enamel products made by cell-free protocols have been tested in clinical trials and were shown to be effective in treating early, minor caries (Yamagishi et al., Nature 433:819, 2005; and Brunton et al., Br. Dental J. 215:741, 2013).